Tuning Single‐Molecule Dynamics in Functionalized Mesoporous Silica

Diffusion of single molecules of a substituted terrylene diimide dye in functionalized mesoporous silica films was monitored by single‐molecule fluorescence microscopy. By varying the chemical nature and density of the functional groups, the diffusion dynamics of the dye molecules can be controlled precisely. The picture shows a sketch of a dye molecule in a pore, diffusion data for different phenyl functionalization densities, and the trajectory of one molecule in a cyanopropyl‐functionalized film.

     

Energy Transfer at the Single‐Molecule Level: Synthesis of a Donor–Acceptor Dyad from Perylene and Terrylene Diimides

In 2004, we reported single‐pair fluorescence resonance energy transfer (spFRET), based on a perylene diimide (PDI) and terrylene diimide (TDI) dyad ( 1 ) that was bridged by a rigid substituted para‐terphenyl spacer. Since then, several further single‐molecule‐level investigations on this specific compound have been performed. Herein, we focus on the synthesis of this dyad and the different approaches that can be employed. An optimized reaction pathway was chosen, considering the solubilities, reactivities, and accessibilities of the building blocks for each individual reaction whilst still using established synthetic techniques, including imidization, Suzuki coupling, and cyclization reactions. The key differentiating consideration in this approach to the synthesis of dyad 1 is the introduction of functional groups in a nonsymmetrical manner onto either the perylene diimide or the terrylene diimide by using imidization reactions. Combined with well‐defined purification conditions, this modified approach allows dyad 1 to be obtained in reasonable quantities in good yield.     

Interplay of Hole Transfer and Host–Guest Interaction in a Molecular Dyad and Triad: Ensemble and Single‐Molecule Spectroscopy and Sensing Applications

A new molecular dyad consisting of a Cy5 chromophore and ferrocene (Fc) and a triad consisting of Cy5, Fc, and β‐cyclodextrin (CD) are synthesized and their photophysical properties investigated at both the ensemble and single‐molecule levels. Hole transfer efficiency from Cy5 to Fc in the dyad is reduced upon addition of CD. This is due to an increase in the Cy5‐Fc separation (r) when the Fc is encapsulated in the macrocyclic host. On the other hand, the triad adopts either a Fc‐CD inclusion complex conformation in which hole transfer quenching of the Cy5 by Fc is minimal or a quasi‐static conformation with short r and rapid charge transfer. Single‐molecule fluorescence measurements reveal that r is lengthened when the triad molecules are deposited on a glass substrate. By combining intramolecular charge transfer and competitive supramolecular interaction, the triad acts as an efficient chemical sensor to detect different bioactive analytes such as amantadine hydrochloride and sodium lithocholate in aqueous solution and synthetic urine.     

Spectral Diffusion of Single Dibenzoterrylene Molecules in 2,3‐Dimethylanthracene

We study single dibenzoterrylene molecules embedded in the dipolar disordered crystal 2,3‐dimethylanthracene at 1.25 K. Broad linewidths (about 1 GHz, ～30 times broader than in the anthracene crystal), high saturation excitation intensities (～1000 times larger than in anthracene), as well as strong spectral diffusions are observed. Additionally, spectral jumping is studied by varying the excitation intensity and the temperature. We propose that the spectral diffusion and dynamic disorder in this system arise from the combination of a static disorder with slight reorientations of the methyl groups of the host molecules.     

Spectroscopy of Single Dibenzoterrylene Molecules in para‐Dichlorobenzene

We study single dibenzoterrylene (DBT) molecules embedded in 1,4‐dichlorobenzene (para‐dichlorobenzene, pDCB) at 1.2 K. Due to the relatively low melting point of pDCB (53 °C), this host‐guest system can be easily prepared from the molten phase. Narrow linewidths, stable molecular lines and high saturation count rates of single DBT molecules were observed. For this reason, we consider this host‐guest system a promising candidate for the study of interactions of single molecules with other small objects such as waveguides or nanoparticles.     

Single‐Molecule Spectroscopy on a Ladder‐Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low‐temperature single‐molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl‐substituted ladder‐type poly(para‐phenylene) (MeLPPP) to investigate the electron–phonon coupling to low‐energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron–phonon coupling to low‐frequency vibrations of the surrounding matrix of the chromophores, and that low‐energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line‐broadening mechanism of the zero‐phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.     

Cell‐Derived Vesicles for Single‐Molecule Imaging of Membrane Proteins

A new approach is presented for the application of single‐molecule imaging to membrane receptors through the use of vesicles derived from cells expressing fluorescently labeled receptors. During the isolation of vesicles, receptors remain embedded in the membrane of the resultant vesicles, thus allowing these vesicles to serve as nanocontainers for single‐molecule measurements. Cell‐derived vesicles maintain the structural integrity of transmembrane receptors by keeping them in their physiological membrane. It was demonstrated that receptors isolated in these vesicles can be studied with solution‐based fluorescence correlation spectroscopy (FCS) and can be isolated on a solid substrate for single‐molecule studies. This technique was applied to determine the stoichiometry of α3β4 nicotinic receptors. The method provides the capability to extend single‐molecule studies to previously inaccessible classes of receptors.     

Ortho‐Dichlorobenzene Doped with Terrylene—a Highly Photo‐Stable Single‐Molecule System Promising for Photonics Applications

The study of a new dye‐matrix system—quickly frozen ortho‐dichlorobenzene weakly doped with terrylene—via single‐molecule (SM) spectroscopy is presented. The spectral and photo‐physical properties, dynamics, and temperature broadening of SM spectra at low temperatures are discussed. The data reveal a broad inhomogeneous distribution, which indicates a high degree of matrix inhomogeneities, but at the same time, huge fluorescence emission rates and extraordinary SM spectral and photochemical stability with almost complete absence of blinking and bleaching. These unusual properties render the new system a promising candidate for applications in photonics, for example, for delivering single photons on demand.     

Investigating the Photostability of Quantum Dots at the Single‐Molecule Level

Quantum dots (QDs) have shown great potential to provide spatial, temporal, and structural information for biological systems. However, blinking, photobleaching, and spectral blueshift are adverse effects on their practical applications in biomedical research. An investigation of the effects of six reducing agents including cysteine (Cys), 1,4‐dithiothreitol (DTT), ethyl gallate (EG), L ‐glutathione (GSH), mercaptoacetic acid (MAA), and thiourea (TU) on the photostability of single QDs was studied. Our experiments demonstrate that both DTT and EG effectively inhibit blinking, photobleaching, and spectral blueshift. GSH molecules block blinking and photobleaching of QDs. The other reagents, Cys, MAA, and TU, only have the ability to counteract blinking. Possible explanations are given on the basis of research evidence. The results suggest possibilities for significant improvements in QDs for biological applications by adjusting the environmental conditions.     

Determination of the Superradiance Coherence Length of Directly Linked Linear Porphyrin Arrays at the Single‐Molecule Level

Down to the wire : Photobleaching dynamics show the exciton delocalization length of directly linked porphyrin arrays (see picture) to be about four or five porphyrin units at the single‐molecule level. This result provides a better understanding of how light‐signal transmission occurs in the solid state and gives a perspective for the porphyrin arrays to be used as single‐molecule photonic wires.

     

Variable Temperature Single‐Molecule Dynamics of MEH‐PPV

Herein, we continue our investigation of the single‐molecule spectroscopy of the conjugated polymer poly[2‐methoxy,5‐(2′‐ethylhexyloxy)‐p‐phenylene‐vinylene] (MEH‐PPV) at cryogenic temperatures. First, the low temperature microsecond dynamics of single MEH‐PPV conjugated polymer molecules are compared to the dynamics at room temperature revealing no detectible temperature dependence. The lack of temperature dependence is consistent with the previous assignment of the dynamics to a mechanism that involves intersystem crossing and triplet–triplet annihilation. Second, the fluorescence spectra of single MEH‐PPV molecules at low temperature are studied as a function of excitation wavelength (i.e. 488, 543, and 568 nm). These results exhibit nearly identical fluorescence spectra for different excitation wavelengths. This strongly suggests that electronic energy transfer occurs efficiently to a small number of low‐energy sites in the multichromophoric MEH‐PPV chains.     

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single‐Molecule‐Magnet Nodes within a Metal–Organic Framework

Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three‐dimensional array. Herein, we show that isonicotinic acid N‐oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy₂ single‐molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single‐crystal to single‐crystal transformation between the phases Dy₂(INO)₄(NO₃)₂?2 solvent (solvent=DMF (Dy₂‐DMF), CH₃CN (Dy₂‐CH₃CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy₂‐DMF and 76 cm^?1 for Dy₂‐CH₃CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy₂ nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.     

Identifying Multiple Populations from Single‐Molecule Lifetime Distributions

A major advantage of single‐molecule methods over ensemble‐averaging techniques involves the ability to characterize heterogeneity through the identification of multiple molecular populations. It can be challenging, however, to determine absolute values of dynamic parameters (and to relate these values to those determined from a conventional method) because characteristic timescales of various populations may vary over many orders of magnitude, and under a given set of experimental conditions instrumental sensitivity to various populations may be unequal. Using data obtained from the single‐molecule tracking microscopy of fibrinogen protein adsorption and desorption, it is shown that by performing a combined analysis of molecular trajectories obtained using a range of acquisition times, it is possible to extract quantitative absolute values of multiple population fractions and residence times (with well‐defined uncertainties), even when these values span many orders of magnitude. In particular, as many as six distinct populations are rigorously identified, exhibiting characteristic timescales that vary over nearly three orders of magnitude with population fractions as small as one part in a thousand. This approach will lead to better comparability between single‐molecule experiments and may be useful in connecting single‐molecule to ensemble‐averaged observations.